The present invention relates to a photographic zoom lens system. More particularly, the present invention relates to a telephoto zoom lens system of a four-group composition that is simple and compact in structure and which yet achieves good performance with a zoom ratio of about 4.
A telephoto zoom lens system that is composed, in order from the object side, of a positive, negative, positive and positive lens group (a total of four lens groups) and which provides a high zoom ratio by moving all lens groups is described in prior patents including Japanese Unexamined Published Patent Application Nos. 78319/1985 and 133916/1986. This type of lens system has the advantage that it is capable of providing a large zoom ratio as compared with a zoom lens system of the so-called "four-group" type which performs zooming with the negative second lens group and the positive third lens group being moved as a variator and a compensator, respectively. However, in the lens system that performs zooming by moving all lens groups it is also required to move a diaphragm stop during zooming, and this causes a disadvantage in that the diaphragm stop mechanism becomes complicated.
On the other hand, the diaphragm stop mechanism does not become complicated if the fourth lens group is fixed. However, in this type of zoom lens system, the diaphragm stop is situated in the rear part (i.e., fourth lens group) of the lens system and the purpose of improving the lens performance and ensuring sufficient marginal light when the system is stopped down cannot be attained unless the diameter of the front lens is increased.